Methods and apparatus for detection of large leaks in sealed articles

ABSTRACT

Methods and apparatus are provided for detection of large leaks in scaled articles. Apparatus for leak detection includes a first sealable chamber configured to receive a test piece containing a trace gas, a second sealable chamber, a first valve coupled between the first and second chambers, a leak detector having a test port, a trace gas permeable member coupled between the second chamber and the test port of the leak detector, a vacuum pump having an inlet, and a second valve coupled between the second chamber and the inlet of the vacuum pump. The permeable member may be quartz, which is permeable to helium when heated. The leak detector may be an ion pump or a helium mass spectrometer leak detector.

FIELD OF THE INVENTION

This invention relates to detection of leaks in sealed articles and,more particularly, to methods and apparatus for detection of large leaksin hermetically sealed articles with small internal volumes.

BACKGROUND OF THE INVENTION

Helium mass spectrometer leak detection is a well-known leak detectiontechnique. Helium is used as a tracer gas which passes through thesmallest of leaks in a sealed test piece. After passing through a leak,a test sample containing helium is drawn into a leak detectioninstrument and is measured. An important component of the instrument isa mass spectrometer tube which detects and measures the helium. Theinput test sample is ionized and mass analyzed by the spectrometer tubein order to separate the helium component. In one approach, a test pieceis pressurized with helium. A sniffer probe connected to the test portof the leak detector is moved around the exterior of the test piece.Helium passes through leaks in the test piece, is drawn into the probeand is measured by the leak detector. In another approach, the interiorof the test piece is coupled to the test port of the leak detector andis evacuated. Helium is sprayed onto the exterior of the test piece, isdrawn inside through a leak and is measured by the leak detector.

One of the difficulties associated with helium mass spectrometer leakdetection is that the inlet of the mass spectrometer tube must bemaintained at a relatively low pressure, typically 2×10⁻⁴ Torr. In aso-called conventional leak detector, the test port, which is connectedto the test piece or to the sniffer probe, must be maintained atrelatively low pressure. Thus, the vacuum pumping cycle is relativelylong. Furthermore, in the testing of leaky or large volume parts, it maybe difficult or impossible to reach the required pressure level. If therequired pressure level can be reached, the pumping cycle is lengthy.

Techniques have been proposed in the prior art to overcome thisdifficulty. A counterflow leak detector disclosed in U.S. Pat. No.3,690,151, issued Sep. 12, 1972 to Briggs, utilizes a technique ofreverse flow of helium through a diffusion pump to the massspectrometer. The leak detector test port can be operated at thepressure of the diffusion pump foreline. A similar approach utilizesreverse flow of helium through a turbomolecular pump. A technique forgross leak detection is disclosed in U.S. Pat. No. 4,735,084 issued Apr.5, 1988 to Fruzzetti. The tracer gas is passed in reverse directionthrough one or two stages of a mechanical vacuum pump. These techniqueshave permitted the test port pressure to be higher than for conventionalleak detectors. Nonetheless, reaching the higher test port pressure canbe difficult when testing large volumes, dirty parts or parts with largeleaks.

In conventional helium leak detection, where a large leak is present ina hermetically sealed small part, the helium can be pumped away so fastduring the rough pump cycle that no leak reading is possible and theleaking part is accepted. This problem has existed in the industry for along time. The following methods have been utilized for someapplications with limited results: (1) measure the difference inevacuation time between a leaky part and a non-leaky part, and (2) avolumetric expansion method. Neither technique provides sufficientresolution. U.S. Pat. No. 5,625,141, issued Apr. 29, 1997 to Mahoney etal., discloses a helium mass spectrometer leak detector combined with avolume expansion technique for gross leak detection.

European Patent Application No. 0 352 371 published Jan. 31, 1990discloses a helium leak detector including an ion pump connected to aprobe in the form of a silica glass capillary tube. The silica glasstube is heated to a temperature between 300° C. and 900° C. and therebybecomes permeable to helium. U.S. Pat. No. 5,325,708 issued Jul. 5, 1994to De Simon discloses a helium detecting unit using a quartz capillarymembrane, a filament for heating the membrane and an ion pump. U.S. Pat.No. 5,661,229 issued Aug. 26, 1997 to Bohm et al. discloses a leakdetector with a polymer or heated quartz window for selectively passinghelium to a gas-consuming vacuum gauge.

All of the prior art helium leak detectors have had one or moredrawbacks, including limited pressure ranges, susceptibility tocontaminants and/or high cost. Accordingly, there is a need for improvedmethods and apparatus for leak detection.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, apparatus for leakdetection is provided. The apparatus comprises a first sealable chamberconfigured to receive a test piece containing a trace gas, a secondsealable chamber, a first valve coupled between the first and secondchambers, a leak detector having a test port, a trace gas permeablemember coupled between the second chamber and the test port of the leakdetector, a vacuum pump having an inlet, and a second valve coupledbetween the second chamber and the inlet of the vacuum pump.

The permeable member may be permeable to helium, and the trace gaspermeability of the permeable member may be controllable. In someembodiments, the permeable member comprises a quartz member. Theapparatus may further comprise a heating element in thermal contact withthe quartz member and a controller configured to control the heatingelement.

According to a second aspect of the invention, apparatus for leakdetection is provided. The apparatus comprises a first sealable chamberconfigured to receive a test piece containing a trace gas, a secondsealable chamber, a first valve coupled between the first and secondchambers, a leak detector including a test port and a vacuum pump, asecond valve coupled between the second chamber and the test port of theleak detector, and a trace gas permeable member coupled between thesecond chamber and the test port of the leak detector.

According to a third aspect of the invention, a method for leakdetection is provided. The method comprises providing a first sealablechamber, a second sealable chamber and a first valve coupled between thefirst and second chambers, placing a test piece containing a trace gasin the first chamber with the first valve closed, vacuum pumping thesecond chamber with the first valve closed, opening the first valve,wherein gas in the first chamber expands into the second chamber,providing a trace gas permeable member coupled to the second chamber,and detecting a leak in the test piece by sensing the trace gas thatpassed through the permeable member.

According to a fourth aspect of the invention, apparatus for leakdetection is provided. The apparatus comprises a first sealable chamberconfigured to receive a test piece containing a trace gas, a secondsealable chamber, a first valve coupled between the first and secondchambers, a first leak detector including a test port and a vacuum pump,a second valve coupled between the second chamber and the test port ofthe first leak detector, a second leak detector having a test port, anda trace gas permeable member coupled between the second chamber and thetest port of the second leak detector.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the accompanying drawings, which are incorporated herein by referenceand in which:

FIG. 1 is a schematic block diagram of leak detection apparatus inaccordance with a first embodiment of the invention;

FIG. 1A is a simplified, partial cross-sectional diagram of the leakdetection apparatus of FIG. 1, showing the permeable member;

FIG. 2 is a schematic block diagram of leak detection apparatus inaccordance with a second embodiment of the invention;

FIG. 3 is a schematic block diagram of leak detection apparatus inaccordance with a third embodiment of the invention;

FIG. 4 is a simplified flow chart of a method for leak detection inaccordance with an embodiment of the invention; and

FIG. 5 is a schematic block diagram of a prior art leak detector.

DETAILED DESCRIPTION OF THE INVENTION

A schematic block diagram of leak detection apparatus in accordance witha first embodiment of the invention is shown in FIG. 1. A first sealablechamber 10 holds a test piece 12. The internal volume of test piece 12is pressurized with helium or is exposed to a high helium concentrationbefore being inserted into the first chamber 10 of the leak detectionapparatus. A second sealable chamber 14 is connected to first chamber 10through a first valve 16. A vacuum pump 20 having an inlet 21 isconnected through a second valve 22 to second chamber 14. Vacuum pump 20may be any type able to evacuate down to a pressure of about 100millibar.

A helium detector assembly 30 is connected via a vacuum flange 32 tosecond chamber 14. Helium detector assembly 30 includes an ion pump 34,an ion pump controller 36 and a trace gas permeable member 40. Ion pump34 and permeable member 40 are mounted in a sealed housing 42 withpermeable member 40 interposed between second chamber 14 and ion pump34. Controller 36 is connected to ion pump 34 via a vacuum feedthrough38. Controller 36 supplies power to ion pump 34 and senses ion pumpcurrent.

Ion pump 34 is typically energized by a high voltage, between 2000 and9000 volts, supplied by controller 36. The ion pump current isproportional to the vacuum pressure inside the ion pump. Helium thatpermeates through permeable member 40 affects the vacuum pressure at arate that is proportional to the leak rate. The ion pump current istherefore proportional to the leak rate.

Trace gas permeable member 40 is located between second chamber 14 andion pump 34. Permeable member 40 is a material that is permeable to thetrace gas used in the leak detection apparatus, typically helium, underspecified conditions. Permeable member 40 substantially passes, orpermeates, the trace gas while substantially blocking other gases,liquids and particles. The permeable member 40 thus acts as a trace gaswindow in the sense of allowing the trace gas to pass while blockingother gases, liquids and particles. Permeable member 40 may have theshape of a disk, for example.

Quartz, or silica glass, is an example of a material that is permeableto helium. In particular, the helium permeability of quartz varies withtemperature. At elevated temperatures in the range of 300° C. to 900°C., quartz has a relatively high helium permeability. At roomtemperature, quartz has a relatively low helium permeability. As shownin FIG. 1A, the leak detection apparatus may be provided with a heatingelement 50 in thermal contact with quartz permeable member 40. Theheating element heats the quartz material to increase heliumpermeability while the quartz selectively blocks most other gases, watervapor and particles. The quartz has a constant permeability for a giventemperature. The temperature can be adjusted to control the permeabilityand therefore the sensitivity. Heating element 50 may be energized by acontroller 52. By controlling the temperature of permeable member 40, ahelium window is provided. At a relatively high temperature (e.g., 300°C. to 900° C.), helium permeability is high and the helium window isopen. At a relatively low temperature (e.g., room temperature), heliumpermeability is low and the helium window is closed. Permeable member 40may be heated by resistive heating, radiant heating, or any othersuitable heating technique.

Permeable member 40 can be made of any suitable material that ispermeable to the trace gas, typically helium, and may have any shape ordimension. Examples of suitable materials include quartz and permeablepolymers such as tetrafluoroethylene, known under the trade name Teflon.The heating element is not required in the case of a permeable polymer.The permeable member can operate at vacuum, at atmospheric pressure orat a pressure slightly higher than atmospheric pressure. The permeablemember can operate in an atmosphere that contains gases, particles andin wet environments.

In the embodiment of FIG. 1, vacuum pump 20 is utilized alone and is notpart of a leak detector. In this embodiment, a fine leak test is notperformed after a large leak test.

A schematic block diagram of leak detection apparatus in accordance witha second embodiment of the invention is shown in FIG. 2. Like elementsin FIGS. 1 and 2 have the same reference numerals. In the embodiment ofFIG. 2, a test port of a leak detector 24 is connected through secondvalve 22 to second chamber 14. In the embodiment of FIG. 2, vacuum pump20 is omitted and leak detector 24 includes a suitable vacuum pump thatis connectable through valve 22 to second chamber 14. In thisembodiment, a fine leak test may optionally be performed after the largeleak test. Leak detector 24 may be any leak detector which includes avacuum pump. An example of a suitable leak detector is shown in FIG. 5and is described below. However, the leak detector is not limited to theexample shown in FIG. 5.

Chambers 10 and 14 are interconnected by first valve 16, and vacuum pump20 is connected to second chamber 14 through second valve 22. The heliumdetector assembly 30 is connected to second chamber 14. With first valve16 closed to isolate chambers 10 and 14 from each other, the test piece12 is placed in first chamber 10. At the beginning of the test, thehelium concentration in first chamber 10 is at ambient level oralternately, a nitrogen flushing operation can be used to reduce thehelium concentration in order to enhance sensitivity for the large leakmeasurement.

With first valve 16 closed, second valve 22 is opened to vacuum pump 20so as to pump second chamber 14 to a desired vacuum level. Valve 22 isthen closed so there is no vacuum pumping of either chamber. Valve 16between chambers 10 and 14 is then opened, and the gas in chamber 10 ispermitted to flow into chamber 14 until pressure equilibrium isachieved. Helium leaking from test piece 12 passes into second chamber14 and increases the helium concentration in the vacuum environmentuntil a pressure equilibrium is reached. The resulting heliumconcentration in second chamber 14 can now be measured.

Only helium passes through permeable member 40 and increases thepressure in ion pump 34. The increase in helium pressure produces anincrease in ion pump current which is proportional to the increase inhelium pressure and to the leak rate. The helium detector assembly 30has essentially zero pumping speed in second chamber 14, except forhelium that passes through permeable member 40, and does not remove gasfrom second chamber 14 as in the case of prior art devices. The heliumdetector assembly 30 detects the helium leak but does not pump thehelium away, so large leaks are detected more accurately, more reliablyand with more sensitivity than prior art methods.

In the embodiment of FIG. 1, the process is complete after measuring theion pump current and determining the presence or absence of a largeleak. In the embodiment of FIG. 2 where leak detector 24 is connectedthrough second valve 22 to second chamber 14, the leak detector 24 canbe utilized to detect a small leak if no large leak is detected byhelium detector assembly 30. Second valve 22 is opened, and secondchamber 14 is pumped to a pressure level suitable for operation of leakdetector 24. The leak detector 24 is then utilized to detect thepresence or absence of a small leak in test piece 12.

A schematic block diagram of leak detection apparatus in accordance witha third embodiment of the invention is shown in FIG. 3. Like elements inFIGS. 2 and 3 have the same reference numerals. In the embodiment ofFIG. 3, ion pump 34 and ion pump controller 36 are omitted, and housing42 is connected by a conduit 50 to the test port of leak detector 24.

To perform a large leak test, test piece 12 is placed in first chamber10, and first valve 16 is closed. Second valve 22 is opened, and secondchamber 14 is vacuum pumped with the vacuum pump that is part of leakdetector 24. Then second valve 22 is closed and first valve 16 isopened. This allows the pressure to equalize between first chamber 10and second chamber 14. Helium that leaks from test piece 12 passesthrough permeable member 40, housing 42 and conduit 60 to leak detector24. The helium is detected by leak detector 24, and the presence orabsence of a leak is determined. Because second valve 22 is closedduring large leak detection, the pressure in second chamber 14 ismaintained, except for the helium that passes through permeable member40. As a result, helium is not rapidly pumped away and can be detected.

A simplified flow chart of a method for a leak detection in accordancewith an embodiment of the invention is shown in FIG. 4. The method isdescribed with reference to the leak detection apparatus shown in FIGS.1-3 and described above. In step 100, test piece 12 is placed in firstchamber 10. In step 102, valve 16 between first chamber 10 and secondchamber 14 is closed. Then, valve 22 is opened, and second chamber 14 isvacuum pumped in step 104. In step 106, valve 22 is closed and valve 16between the first and second chambers is opened. This allows the gas infirst chamber 10 to expand into second chamber 14, thereby equalizingthe pressure in the first and second chambers. If the apparatus includesheating element 50 as shown in FIG. 1A, the heating element maybeenergized to increase the helium permeability of permeable member 40. Instep 108, the helium in second chamber 14 is sensed with second valve 22closed. In the embodiments of FIGS. 1 and 2, helium is sensed in step108 by ion pump 34. In the embodiment of FIG. 3, helium is sensed instep 108 by leak detector 24. In step 110, a determination is made as towhether a large leak is present in test piece 12, based on the sensedhelium that passes through permeable member 40. Large leak detectionstep 110 completes the process in the embodiment of FIG. 1. In theembodiments of FIGS. 2 and 3, valve 22 is opened in step 112 and secondchamber 14 is vacuum pumped to a lower pressure level to permit smallleak detection. In embodiments which include heating element 50 forheating permeable member 40, the heating element may be de-energized instep 112. In step 114, a leak test is performed by leak detector 24, andthe presence or absence of a small leak is detected in step 116.

An example of a prior art leak detector suitable for use in the leakdetection apparatus of FIGS. 2 and 3 is shown in FIG. 5. A test port 230is coupled through a roughing valve 232 to a roughing pump 234. The testport 230 is also coupled through a test valve 236 to the foreline 238 ofa high vacuum pump 240. Vacuum pump 240 may be a turbomolecular pump, adiffusion pump or a hybrid turbomolecular pump which includes axialpumping stages and molecular drag stages. The foreline 238 is alsocoupled to a forepump 242 which maintains the required operatingpressure at the foreline 238. The inlet of vacuum pump 240 is coupled tothe inlet of a mass spectrometer tube 244.

In operation, the roughing pump 234 initially evacuates the test port230 and second chamber 14 to a pressure in the range of 100 to 300millitorr. The test valve 236 is then opened and the helium tracer gasdrawn in through the test port 230 passes in reverse direction throughvacuum pump 240 to the spectrometer tube 244. Since the vacuum pump 240has a much lower reverse flow rate for the heavier gases in the sample,it blocks these gases from spectrometer tube 244, thereby efficientlyseparating the tracer gas.

Having thus described various illustrative non-limiting embodiments, andaspects thereof, modifications and alterations will be apparent to thosewho have skill in the art. Such modifications and alterations areintended to be included in this disclosure, which is for the purpose ofillustration and explanation, and not intended to define the limits ofthe invention. The scope of the invention should be determined fromproper construction of the appended claims and equivalents thereof.

1. Apparatus for leak detection comprising: a first sealable chamberconfigured to receive a test piece containing a trace gas; a secondsealable chamber; a first valve coupled between the first and secondchambers; a leak detector having a test port; a trace gas permeablemember coupled between the second chamber and the test port of the leakdetector; a vacuum pump having an inlet; and a second valve coupledbetween the second chamber and the inlet of the vacuum pump. 2.Apparatus as defined in claim 1, wherein the leak detector comprises anion pump.
 3. Apparatus as defined in claim 1, wherein the permeablemember is permeable to helium.
 4. Apparatus as defined in claim 1,wherein the permeable member comprises a quartz member, the apparatusfurther comprising a heating element in thermal contact with the quartzmember and a controller configured to control the heating element. 5.Apparatus for leak detection comprising: a first sealable chamberconfigured to receive a test piece containing a trace gas; a secondsealable chamber; a first valve coupled between the first and secondchambers; a leak detector including a test port and a vacuum pump; asecond valve coupled between the second chamber and the test port of theleak detector; and a trace gas permeable member coupled between thesecond chamber and the test port of the leak detector.
 6. Apparatus asdefined in claim 5, wherein the second valve is closed at relativelyhigh pressures in the second chamber and wherein the second valve isopen at relatively low pressures in the second chamber.
 7. Apparatus asdefined in claim 5, wherein the permeable member comprises a quartzmember, the apparatus further comprising a heating element in thermalcontact with the quartz member and a controller configured to controlthe heating element.
 8. Apparatus as defined in claim 5, wherein a tracegas permeability of the permeable member is controllable.
 9. Apparatusas defined in claim 5, wherein the permeable member is permeable tohelium.
 10. A method for leak detection, comprising: providing a firstsealable chamber, a second sealable chamber and a first valve coupledbetween the first and second chambers; placing a test piece containing atrace gas in the first chamber with the first valve closed; vacuumpumping the second chamber with the first valve closed; opening thefirst valve, wherein gas in the first chamber expands into the secondchamber; providing a trace gas permeable member coupled to the secondchamber; and detecting a leak in the test piece by sensing the trace gasthat passed through the permeable member.
 11. A method as defined inclaim 10, further comprising vacuum pumping the second chamber with thefirst valve open, and sensing the trace gas pumped from the secondchamber to provide detection of small leaks.
 12. A method as defined inclaim 11, further comprising controlling the permeable member at a hightrace gas permeability at relatively high pressures in the secondchamber and controlling the permeable member at a low trace gaspermeability at relatively low pressures in the second chamber.
 13. Amethod as defined in claim 12, wherein controlling the permeable membercomprises heating the permeable member.
 14. A method as defined in claim10, wherein sensing the trace gas that passed through the permeablemember comprises sensing the trace gas with an ion pump and monitoringion pump current.
 15. A method as defined in claim 10, wherein sensingthe trace gas that passed through the permeable member comprises sensingthe trace gas with a leak detector including a mass spectrometer.
 16. Amethod as defined in claim 11, wherein sensing the gas pumped from thesecond chamber comprises sensing the trace gas with a leak detector. 17.Apparatus for leak detection comprising: a first sealable chamberconfigured to receive a test piece containing a trace gas; a secondsealable chamber; a first valve coupled between the first and secondchambers; a first leak detector including a test port and a vacuum pump;a second valve coupled between the second chamber and the test port ofthe first leak detector; a second leak detector having a test port; anda trace gas permeable member coupled between the second chamber and thetest port of the second leak detector.
 18. Apparatus as defined in claim17, wherein the second valve is closed at relatively high pressures inthe second chamber and wherein said second valve is open at relativelylow pressures in the second chamber.
 19. Apparatus as defined in claim17, wherein the permeable member comprises a quartz member, theapparatus further comprising a heating element in thermal contact withthe quartz member and a controller configured to control the heatingelement.
 20. Apparatus as defined in claim 17, wherein the trace gaspermeability of the permeable member is controllable.
 21. Apparatus asdefined in claim 17, wherein the permeable member is permeable tohelium.
 22. Apparatus as defined in claim 17, wherein the second leakdetector comprises an ion pump.